We are interested in transcription factors that function in the regulation of cell fate determination during development. Our model system is the nematode C. elegans (a non-parasitic worm) that is widely used for developmental studies because of its small size, ease of culture in the laboratory, simple anatomy, rapid proliferation, and genetics. We are currently interested in several transcription factors that have been identified in other systems as important for mesoderm patterning and muscle formation. By studying the phenotypes that result from mutations in these genes we are beginning to define their exact roles in regulating the development of specific subsets of muscle cells in C. elegans.[unreadable] In mice, striated muscle development requires the function of the MRFs, a set of four closely related factors (MyoD, Myf-5, MRF-4, Myogenin) that regulated muscle specification and differentiation. C. elegans has a single MRF-related factor called CeMyoD. One of the puzzling aspects of C. elegans myogenesis was the observation that muscles could be formed in the absence of CeMyoD, as demonstrated in mutant animals. To further define the exact role of this transcription factor, we ectopically expressed CeMyoD in the early nematode embryo and found that it was sufficient to convert all early blastomeres to a muscle-like fate. The sufficiency of CeMyoD alone to direct cells into the muscle lineage illustrated its potency and revealed a level of evolutionary conservation in function that had not previously been appreciated. Moreover, these experiments revealed a remarkable degree of plasticity of the early embryonic cells to be reprogrammed with regards to cell fate choice.[unreadable] We have used our ability to convert all cells in the C. elegans embryo into muscle to profile gene expression during myogenic differentiation. Combined with studies from other investigators, this analysis suggested that two additional transcription factors (UNC-120, HND-1) might be playing an important role, along with MyoD, in C. elegans muscle development. Using genetic deletion alleles of all three genes, we showed that muscle could form when at least one of these factors was present. However, elimination of all three gene products blocked muscle differentiation in the embryo. These results defined this trio of transcription factors as the key regulators of C. elegans muscle, explaining why muscle could form in the absence of MyoD. [unreadable] In collaboration with the Hanover Lab in NIDDK, we have continued to explore the role of nutrient sensing in development. Using knockout alleles of genes in C. elegans that regulate the addition and removal of sugar residues on nucleocytoplasmic proteins, we found that the hexosamine pathway influences the developmental decision of growth versus diapause in C. elegans. C. elegans may offer a facile genetic system to tease apart the subtle roles of the hexosamine pathway in normal development and disease.